1. Technical Field of the Invention
The present invention relates generally to a gas sensor element for use in measuring the concentration of hydrogen-containing gasses including hydrogen atom-made compounds such as hydrocarbon, hydrogen, and moisture and a measuring method using the same.
2. Background Art
The air population arising from automobile exhaust fumes has posed serious problems with modern life. The emission regulation, thus, has become severe year by year.
More effective emission purification is expected to be achieved by measuring the concentration of hydrocarbon gas contained in the exhaust emissions and feeding it back to an engine burning control monitoring system or a catalyst monitoring system. Accordingly, some hydrocarbon sensor elements have been proposed in the prior art which are capable of determining the concentration of hydrocarbon gas contained in exhaust emissions of automotive vehicles precisely.
FIG. 12 shows a conventional hydrocarbon sensor element 8.
The hydrocarbon sensor element 8 consists essentially of a measurement gas chamber 951 into which exhaust gasses to be measured are admitted and an oxygen pump cell 92 exposed to the measurement gas chamber 951. Application of voltage to the oxygen pump cell 92 causes oxygen molecules to be pumped out of the measurement gas chamber 951 into the ambient atmosphere or vice versa.
The hydrocarbon sensor element 8 also includes an oxygen monitor cell 93 which works to monitor the concentration of oxygen within the measurement gas chamber 951 and produce an electromotive force indicative thereof. A feedback circuit 929 monitors the output of the oxygen monitor cell 93 and controls the voltage applied to the oxygen pump cell 92 so as to keep the concentration of oxygen within the measurement chamber 951 substantially constant.
The oxygen monitor cell 93 has an electrode 931 exposed to the measurement gas chamber 951 which is made of a material that is highly active to oxidize hydrocarbon, thereby enabling the oxygen monitor cell 93 to measure the amount of oxygen remaining after reaction of the hydrocarbon with the oxygen on the electrode 931.
The hydrocarbon sensor element 8 also includes a sensor cell 94 exposed to the measurement gas chamber 951. The sensor cell 94 has an electrode 941 which is exposed to the measurement gas chamber 951 and made of a material that is hardly active to oxidize the hydrocarbon. This causes the oxygen to be excessive on the electrode 941 as compared with around the electrode 931 of the oxygen monitor cell 93. This excessive oxygen is drained from the measurement gas chamber 951 to a reference gas chamber 952 by applying the voltage to the sensor cell 94, thereby causing an oxygen ion current to be produced as an output of the sensor cell 94 which is proportional to entration of hydrocarbon contained in the measurement gasses (i.e., automobile exhaust gasses).
Reference numbers 911 and 912 denote oxygen ion-conductive solid electrolyte bodies.
The hydrocarbon sensor element 8 is, as apparent from the above discussion, designed to use a difference in oxidation activity between the electrodes to measure the concentration of hydrocarbon as a function of an oxygen ion current arising from a difference in concentration of oxygen between the electrodes, thus resulting in a change in sensor output due to a change in concentration of oxygen contained in the measurement gasses or aging of activity of the electrodes. This problem is common to gas sensors designed to measure the concentration of hydrogen-containing gas containing a hydrogen compound such as hydrogen gas or moisture in the same manner as described above.